A semiconductor wafer is, for example, subjected to an etching process via deposition process (film forming process) using a spin coater or the like. The spin coater is a device for forming a thin film such as an insulating film and a photoresist film on the semiconductor wafer. The semiconductor wafer is rotated and a material liquid is dropped on to a central part of an upper surface (front surface) of the wafer, so that the material liquid is spread over the entire surface by centrifugal force. By doing this, a thin film covers not only the main area (the inner side area) of the upper surface of the wafer but also the outer edge part of the upper surface and the outer end face. Should this film occupying the outer edge of the wafer be allowed to remain, it could be a cause for generating particles, for example, in the process for holding the outer edge. Moreover, the film becomes larger in thickness at the outer edge of the wafer than at the main area due to resistance of the liquid stream. This not only results in inconvenience at the time of polishing the film surface but also tends to cause contamination due to peel-off.
In the case of deposition according to PE-CVD and LP-CVD, it is seldom that the film becomes large in thickness at the outer edge of the film, but there is a possibility that crack occurs when the outer edge of the wafer is accidentally contacted during the transferring process, thus resulting in contamination.
In the field of the present leading LSI device, Cu/low-k is the main stream in order to realize high speed operation. In this technology, copper having high electron mobility is used as metal wiring, and a low dielectric film having a lower dielectric constant than SiO2 (dielectric constant: 4) is used as an interlayer insulating film. However, it is said that the low dielectric film is smaller in mechanical strength than SiO2 and therefore, that the film formed on the outer edge of the wafer is liable to be a cause for occurring contamination in a physical polishing process such as CMP (chemical mechanical polishing).
Such contamination is liable to become the cause for such an inconvenience as short circuit of the wiring, and this often results in reduced yield of product. As the miniaturization of semiconductor is developed, the wiring is reduced in thickness and thus readily subject to the adverse effect of contamination. Therefore, it is required to more strictly restrain the occurrence of contamination.
In view of the above, Japanese Patent Application Laid-Open Nos. H03-268419 and H11-274147 teach that a so-called wet etching technique in which an etching liquid is dropped on to the non-required film formed on the outer edge of a wafer in order to remove the no-required film therefrom after the end of the process for forming a film on the upper surface of the wafer.
However, the method for removing a film on the outer edge of a wafer according to the above-mentioned wet etching technique has such inconveniences that not only the film formed on the outer edge but also the film formed on the main area become brittle by moisture of the etching liquid and a huge cost is required for processing the exhaust liquid. Although it is desirous that the outer end face of the film is formed in a slope configuration in order to disperse stress at the time of physical polishing, the outer end face of the film becomes a sharpened edge-like configuration according to the wet etching technique, thus making it difficult to obtain the slope-like configuration.
In contrast, Japanese Patent Application Laid-Open Nos. H05-82478, H08-279494 and H10-189515 and some others propose a technique for removing the film formed on the outer edge of a wafer in accordance with a-so-called dry etching technique using plasma.
In general, a plasma processing apparatus comprises a pair of electrodes. For example, in a plasma processing apparatus for etching illustrated in FIG. 5 of Japanese Patent Application Laid-Open No. H05-82478, a dielectric is wound on each of a pair of concentric dielectric cylinder to provide a double annular electrode structure. An annular gap is formed between the inner and outer annular electrodes, and this annular gap serves as a gas passage for allowing a processing gas to flow therethrough. By imparting an electric field between the electrodes, the gas passage is turned out to be a plasmatizing space where the processing gas is plasmatized. This plasmatized processing gas is blown off through the entire periphery of the annular gas passage. This makes it possible to etch the entire periphery of the outer edge of the disc-like wafer at a time.
Moreover, in the apparatus illustrated in FIG. 1 of the above-mentioned Laid-Open document No. H05-82478, a pipe bent in the C-shape is received in an annular groove which is formed in the electrode unit and allowed to contact an annular electrode, and then, a coolant as a temperature adjusting medium is flowed into this pipe to cool (temperature adjustment) the electrode.
Moreover, in the apparatus disclosed in the above-mentioned Laid-Open document No. H05-82478, a wafer is sandwichingly held at both the front and back sides thereof by a pair of disc-like chucking means. By this, the main areas at both the front and back surfaces of the wafer are covered and the outer edge of the wafer is exposed. Each chucking means is provided at a peripheral edge thereof with an O-ring. This O-ring is pressed against the boundary area between the main area and the outer edge of the front and back surfaces of the wafer. By this, sealing is provided so that the plasma gas will not flow to the main area. The exposed outer edge of the wafer is faced with the interior of the annular closed clearance. By supplying the plasmatized processing gas into this closed clearance, the film formed on the outer edge of the wafer is removed.
Moreover, in the apparatus disclosed in the above-mentioned Laid-Open document No. H05-82478, an exhaust means is separately employed from the above-mentioned processing gas blow-off means. This exhaust means includes an annular suction port disposed proximate to the outer edge of the wafer and connected to the annular closed clearance, and an exhaust passage connected to the annular suction port. The processing gas and the byproduct are sucked through the suction port and exhausted through the exhaust passage.
The apparatus disclosed in the above-mentioned Japanese Patent Application Laid-Open No. H08-279494 includes an annular electrode structure and therefore, an annular plasma blow-off port. The wafer is disposed below the annular blow-off port such that the outer edge of the wafer extends along the port. An electric field is imparted between the annular electrodes, so that the processing gas passing between the electrodes is plasmatized and the plasmatized processing gas is blown off through the annular blow-off port. The plasmatized gas thus blown off is contacted with the outer edge part of the upper surface of the wafer, then traveled downward along the outer edge surface and turned toward the back side. By this, the film formed on the outer edge (including the outer edge part of the upper surface, the outer end face and the outer edge part of the lower surface) of the wafer is removed. Thereafter, the gas is sucked and exhausted from the lower part of the wafer. By separately spraying a carrier gas on to a central area of the wafer and radially spreading the gas, the plasma sprayed on to the outer edge of the wafer is prohibited from flowing to the inner side (i.e., main area of the wafer).
In the apparatus, disclosed in the Japanese Patent Application Laid-Open No. H10-189515, the electrode structure and thus, the plasma blow-off port are directed upward and arranged at a lower part of the outer edge of the wafer.
The conventional plasma processing apparatus, particularly, the plasma processing apparatus comprising an annular electrode structure has the following problems.
(a) Assembling/Centering of Annular Electrode
Electrodes are often required to be attached/detached for cleaning when they become dirty, or for replacement when they are damaged. If some clearance is not formed between each electrode and the holder at that time, the detaching/assembling operation is difficult to carry out. However, if some clearance should be provided in a double annular electrode structure, precision of the attaching position of the electrodes would readily be disordered and the centers of the inner and outer electrodes would be displaced from each other.
(b) Cooling (Temperature Adjustment) of Annular Electrode
In order to occur a glow discharge in a stable manner and thus, in order to carry out the plasmatization (conversion of a processing gas into plasma) in a stable manner, the electrodes are required to be maintained in a predetermined temperature range by means of such temperature adjustment as cooling. On the other hand, in case the electrodes are annular, it is not easy to form a cooling passage for temperature adjustment at the inside of the electrodes. According to the apparatus disclosed in the above-mentioned Laid-Open document No. H05-82478, manufacture is easy because the cooling structure (temperature adjustment structure) is separately formed from the electrodes. However, heat exchange is carried out between the coolant and the electrode through a tube wall of the coolant tube. In case the contact area with the electrode is small, fully satisfactory cooling efficiency (temperature adjusting efficiency) is unobtainable.
Moreover, for example, in the double annular electrode structure, in case a coolant tube extending in the inner peripheral surface is disposed at the inner peripheral surface of the inner electrode and a coolant tube extending in the peripheral direction is disposed at the outer peripheral surface of the outer electrode, if the coolant tubes are unchanged in curvature along the peripheral surfaces of the electrodes and non-expansible/non-contractible, the disassembling/assembling operation is encountered with difficulty at the time of maintenance. On the other hand, it is required that the electrodes and the coolant tubes are firmly contacted with each other when in use and the heat transfer property is maintained.
(c) Exhaust of Processed Gas and Byproduct
The used processing gas and the byproduct film generated by etching are required to be removed rapidly because if not, they can prevail such adverse effect as staying on the wafer. However, in case the blow-off port and exhaust port of the processing gas are located overly away from each other, the gas flow is difficult to control and an exhaust pump having a large capacity is required. In case an exhaust passage is formed in the frame of the apparatus, there is such a fear that the inner peripheral surface of the frame gets corrosion. Any attempt to make the frame from a corrosion-resistant metal results in high cost.
The apparatus disclosed in the Japanese Patent Application Laid-Open No. H05-82478 has such disadvantages that a chip located at an area sandwichingly held by the O-ring of the pair of chucking means is liable to be damaged, and that the film formed on this area is liable to split, thus generating fine dusts, which can be a cause for contamination. Moreover, since a housing for forming an annular closed clearance is required radially outwardly of the wafer, the apparatus becomes large in size. This makes it difficult to employ as a replacement of the wet etching mechanism of the conventional spin coater.
According to the apparatus disclosed in Japanese Patent Application Laid-Open No. H08-279494, the plasma and the carrier gas are converged at the outer edge of the wafer. This makes it difficult to control the flow of those gases, and thus, it is difficult to remove the film formed on the outer edge with precision. If any attempt is made to stop the carrier gas, there is such a fear that the plasma enters not only the outer edge of the wafer but also the area which is located at the inner side of the outer edge and which is not to be processed. Thus, there is a possibility that even the film formed on the area which is not to be removed is removed.